1. Technical Field
The present invention relates to an interference filter that separates light of a predetermined wavelength from incident light, an optical sensor including the interference filter, and an optical module including the optical sensor.
2. Related Art
There have been known interference filters that transmit or reflect only a predetermined wavelength light from incident light (see JP-A-2009-134027, for example).
JP-A-2009-134027 describes an optical device (interference filter) having a pair of substrates facing each other and having reflection films formed on the surfaces of the substrates that face each other. In the interference filter of this type, a gap is formed between the pair of reflection films, and the dimension of the gap determines the wavelength of light that can be separated.
In the interference filter of this type, it is necessary to maintain the pair of reflection films parallel to each other in order to separate light of a predetermined wavelength with spectral precision. For example, when the substrates are bonded to each other by forming an adhesive layer formed of an adhesive between the substrates, it is disadvantageously difficult to control the thickness of the adhesive layer and hence difficult to maintain the reflection films parallel to each other.
To address the problem, it is conceivable to employ a bonding method for bonding the substrates by forming an adhesive groove into which an adhesive is applied in a bonding surface across which the substrates are bonded and applying an adhesive into the adhesive groove.
When the adhesive applied into the adhesive groove is used to bond the substrates to each other as described above, however, volume shrinkage that occurs when the adhesive hardens induces stress in the adhesive applied portion and the stress warps the substrates.
For example, consider a case where a first substrate and a second substrate are bonded to each other with an adhesive by forming an adhesive groove into which an adhesive is applied in the second substrate, applying an adhesive into the adhesive groove, and overlaying the first substrate on the second substrate so that the groove is closed. When the adhesive hardens and shrinks, induced stress warps the first substrate toward the adhesive groove. When the first substrate warps into the adhesive groove, the balance among the forces acting on the entire first substrate becomes unstable. As a result, the pair of reflection films cannot disadvantageously be maintained parallel to each other. When the pair of reflection films are not maintained parallel to each other, the wavelength of separated light varies depending on the position in the reflection films. For example, separated light passing through a portion where the gap is large has a long wavelength, whereas separated light passing through a portion where the gap is small has a short wavelength. The spectral precision of the interference filter is therefore disadvantageously poor.